To provide an anti-torque device capable of improving the efficiency of a tail rotor at a hovering time and also at a high-speed flying time.

In an anti-torque device includes a skin 1 provided in a portion 1 in the rear periphery of the exit side of a tail rotor duct, a mechanism 2 for changing the shape of the skin, an arithmetic device 3 for arithmetic- controlling the mechanism 2 to an optimal state, a steering device 4 and a body moving sensor 5, a linear actuator 9 is used for the mechanism 2 for changing the skin 1, the skin 1 is fixed to a center material 8 by the linear actuator 9 disposed by keeping a proper interval in the rotational direction of the tail rotor, the arithmetic device 3 elongates/contracts the linear actuator 9, outputs a control signal for changing the shape of the skin 1 to the linear actuator 9 and performs control so as to provide a sharp edge to the exit side of the duct during hovering and a large edge radius R (round edge) thereto during high-speed flying.